This information in this article has been
extracted from: 'Su-27 Fighter: Beginning of a History' by I. Betretdinov, N.
Gordukov, V. Zenkin, V. Antonov and P. Plunsky.

The technical requirements for the initial concept for a proposed fighter,
starting in 1971, included the installation of an onboard 30 mm gun with firing
rate of 3000 rounds per minute and an ammunition capacity of 250 rounds.

The recognised leader in the development of aircraft guns was the Tula KB of
instrumentation (KBP, ex-TcseKB-14). At beginning of the 1970s only one suitable
30 mm aircraft gun was in development – the two-barrel Gast-type TKB-645 (9A623)
gun. Technical data concerning the TKB-645 had been taken as the basis for the
initial arrangement of the gun armament of the planned Su-27. As a result, in
the final version of the technical requirement from the Soviet AF for the Su-27
fighter, approved in summer 1976, the intended onboard gun (designated AO-17А)
was a two-barrel weapon with an ammunition load of 250 rounds. It was intended
to supplement this by two SPPU-30 gun pods each with a flexibly-mounted GSh-301
gun (when available) and 250 rounds.

At an early stage of development, studies showed that because of the large
overall dimensions and weight (115 kg) of the TKB-645, installing it in the
planned location on the Su-27 would cause difficulties. [Ed: it was the same weight as the 20 mm M61A1 and somewhat more compact –
the ammunition supply arrangements were also lighter and more compact.] Furthermore, the SPPU-30 gun pod, originally developed for the Su-24M
bomber, was also bulky and heavy (980 kg), so unsuitable for an air superiority
fighter like the Su-27.

As a result, during the initial design development of the Su-27 by Sukhoi OKB a
search for an alternative gun commenced in parallel with studies on how to
install the TKB-645. Specialists from the 10th Department of the OKB
were in constant dialogue with the leading expert and deputy chief designer of
KBP, V.P. Gryazev, and his understanding of the problems of the OKB allowed him
to find a solution to the problem in the shape of the new single-barrel TKB-687
aircraft gun. One mock-up sample of gun had already been built "in metal", and
this appeared promising.

The TKB-687 aircraft gun (9-A-4071, and after its service acceptance - GSh-301,
i.e. Gryazev, Shipunov, 30 mm, 1 barrel), was developed in KBP to achieve the
maximum ballistic and energy characteristics. In comparison with similar western
aircraft guns it provided a high firing rate, was lighter and had the highest
efficiency (a ratio derived from the rate of fire, the weight of the projectiles
and the weight of the gun). [Ed: see this article on modern
aircraft guns for more details of how their performance and efficiency compare].
It was also easy to manufacture. The following designers assisted V.P.
Gryazev in the development of the GSh-301: V.N. Valuev, B.I. Kuznetcsov, E.V.
Davydov, I.M. Paramonov, V.A. Kuzmin and А.М. Kalinin.

The outline drawings of the new gun were transferred to Sukhoi OKB for them to
study its installation in the Su-27. It was discovered that, unlike the TKB-645,
the single-barrel gun could be easily placed in the space allocated within the
structure. In October 1974 an L81-T10 testbed was constructed at the OKB with a
TKB-687 gun installation, in order to carry out live firing tests. For this
purpose KBP supplied an experimental gun to the OKB. Bench tests were conducted
on the TsceNIITOChMASh firing range by an evaluation group from the OKB. The
first test was very short, consisting of only 50 shots. However, this resulted
in: destruction of the testbed's aluminium skin in the area of the barrel, a
crack on structural ribs and damage to the forward mounting point, and a crack
on the cartridge case ejection chute (the case ejection speed was 100 m/s). It
was discovered that the use of an ammunition box with vertical loading caused
significant problems in stacking the ammunition load and in gun reloading.

After analysis of the test results, work commenced on remedying the defects.
These included work on the gun design and on the airframe in the area of the gun
installation. Simultaneously, a search for a new solution for the ammunition
feed system was conducted in the Brigade of Artillery Arms headed by V.V.
Doronkin. The solution was found unexpectedly as a result of studying the design
of the gun installation of an F-5E fighter, captured in Vietnam and made
available to the OKB. The essence of the idea was to loop the cartridge belt
around guide rails. A wooden testbed provided positive results, and after
further development two patents were granted.

In 1976 the testbed L81-T10 was redesigned to incorporate the new feed system.
Tests were carried out from November 1976 until November 1977. During this
process TKB-687 prototypes №15 and №37 were used. The tests resulted in the
destruction of a cartridge case extraction chute and revealed unreliable
functioning of the feed system because of sagging of the cartridge belt and
jamming of the guide rails. Two versions of a powered cartridge belt fed
assisters - mechanical (spring) and pneumatic were urgently developed. Because
of design problems the mechanical assister was rejected and the installation of
a pneumatic assister was recommended.

In parallel with tests of these improvements, theoretical research was carried
out in the OKB to substantiate the concept of gun armament for the Su-27.
Essential help was provided by experts from NIIAS MAP (Scientific Institute of
Aviation Systems of the Aviation Industries Ministry). As a result of this
research and mathematical modelling carried out by employees of Department №6
from NIIAS, it was shown that in a dogfight a mobile (steerable) gun
installation on a fighter has a big advantage. The efficiency of such an
installation in comparison with a fixed gun was about 3-4 times greater, since
in a dogfight the mobile gun became, in effect, an "all aspect" weapon.

By 1977 work on determining the form of the Su-27 gun armament with the TKB-687
gun was practically finished. It was only necessary to approve those solutions
officially. For this purpose Sukhoi OKB released a proposal concerning the
updating of the technical data of the gun armament of the Su-27 fighter.

On September 7th, 1977 KBP was officially entrusted by the VPK (Military
Industries Complex) to develop a new 30 mm gun intended for installation in the
Su-27 on a steerable mounting, and for the MiG-29 and Yak-41 in a fixed
mounting, and also for the replacement of NR-30 guns on aircraft such as the
Su-17M and to equip gun pods. The development of the gun by KBP was authorised
by a departmental order of the MOP (Ministry of Defence Industries) on September
28th, 1977.

In the autumn of 1977 mock-ups of two versions of the gun installation for the
Su-27 were demonstrated:

- VPU-687 – built-in fixed gun installation
with gun angle installed at 0° to the axis of the aircraft (i.e., firing
straight ahead) and an ammunition load of 200 cartridges contained in a
cartridge box (designed as a steep shaft between frames 17a-18);

- PPU-27 - moving gun installation in the same
layout volume as the VPU-687, with the gun being steerable through +5 ° in the
horizontal plane and 0-15° in the vertical plane, upwards from the axis of the
aircraft, with an ammunition load of 150 cartridges.

The mock-up commission of the Soviet Air Forces recommended the development of
VPU-687 with a subsequent transition to the PPU-27. This solution was accepted
owing to insufficient design work on the mobile installation. According to the
approved schedule of PPU-27 improvement, the conclusion of the installation
improvement tests was planned for the end of 1980. The requirements
specification for the development of a rapid-fire single-barrel 30 mm gun under
the designation 9-A-4071 (TKB-687) was signed in December 1977, and in July 1978
the military provided the requirements specification the PPU-27. The development
of the PPU-27 was charged to the specialised design bureau of the Moscow
Aggregate Factory MAZ "Dzerzhinetsc".

A bottleneck appeared in the development of the PPU-27: electric drive. In view
of its sizeable overall dimensions, the steering mechanism would not fit within
the fuselage where needed, due to the lack of height within the structure.
Essentially a new solution was required, and such solution was found in the form
of a rotary hydromotor which was designed in the OKB with the participation of
N.I.Pilipovich, a specialist from NIIAS. But there were many questions regarding
the functioning of a hydromotor and hydroautomatic valves when subject to the
impact loads involved in gun firing. This required a great deal of additional
research and experimental work. Experts on aerodynamics expressed doubts
regarding the stability and controllability of the aircraft when the gun – and
integral hatch cover – was steered; in their opinion, this would sharply change
the airflow pattern over the right wing of the aircraft. As a result of these
problems work on the PPU-27 was suspended and transferred to the NIR for
possible future modifications to the Su-27 fighter.

During this time the OKB continued with work on the operational development of
the fixed TKB-687 installation. Up until 1979 work was carried out on the
L81-T10 testbed. In 1980 a new version of the testbed was constructed: L81-T10C,
which corresponded to the layout of the production airplane. By September 1981
the first stage of the tests on L81-T10C were concluded, after the firing of
1,480 rounds. As as result of these tests a number of adaptations were made to
the VPU. The strength of the forward gun mounting point and the cartridge case
extraction chute were increased. A second stage of tests, in which 4,000 rounds
were fired, finished by April 1982. Experts from NIIAS took part in this work.

Factory flight tests of the VPU-687 mounting on the Su-27 were conducted in 1982
on the prototype T10-3. The programme of tests was split into two stages, the
first being carried out from June 5th to July 14th, and
the second - from October 10th, till December 9th 1982.
Flights were preceded with ground tests of the VPU involving shooting in a
firing range. The first flight with air firing tests was carried out by test
pilot N.F. Sadovnikov on July 9th, 1982. Test flights with VPU-687 were also
executed by test-pilots from Sukhoi OKB V.G.Pugachev and A.N.Isakov. The design
support of tests was provided by experts of the arms department V.V.Doronkin,
N.A.Remezov and G.I.Sitnikov.

During gun armament test flights the influence of gun firing on the functioning
of the starboard engine, on the fly-by-wire system and on avionics complex as a
whole was checked. Vibration was measured and its effect on equipment assessed,
and the concentration of the air-gas mixture in the compartments of the VPU-687
mounting was measured to determine explosion safety limits. During this period
21 flights were made and 969 shots fired. The efficiency of the gun was
confirmed by shooting up to six bursts with 25 rounds in each. The conclusion of
test-pilot N.F.Sadovnikov, supported by information from flight recorders, was
that firing the GSh-301 did not influence stability, controllability or engine
functioning. The pilots experienced vibration in the cabin, but this did not
cause any instrument failures.

The next stage of gun armament tests were executed during government
certification tests of the Su-27, on series production configuration aircraft,
fully equipped with nominal avionics and equipment.